Method and apparatus for grinding flat glass on both faces simultaneously



Dec. H, E951 J. H. GRlFFlN 2,577,889

METHOD AND APPARATUS FOR GRINDING FLAT GLASS ON BOTH FACES SIMULTANEOUSLY Filed Aug. 26, 1949 Inventor l atented Dec. 11, 1951 METHOD AND APPARATUS FOR GRINDING FLAT GLASS ON BOTH FACESiSIMULTA- NEOUSLY James Harris Grifiin, Moss Bank, St. Helens, England, assignor to Pilkington Brothers Limited, Liverpool, England, a corporationof Great Britain Application August 26, 1949, Serial No. 112,522

This invention relates to methods of and apparatus for grinding flat glass on both faces simultaneously and especially to methods of and apparatus for continuously grinding the underface of horizontally disposed fiat glass in ribbon form simultaneously with the grinding of the upper face.

In grinding glass in horizontal ribbon form, it is usual to grind the ribbon of glass, which is formed from molten glass in a tank, on both faces simultaneously as it travels forward from the lehr, by employing pairs of opposed grinder discs, each rotating. about a vertical axis, between which discs the ribbon moves whilst being supported in the grinding bays by the bottom I grinder discs. 7

In such form of apparatus, each grinder disc commonly comprises an annular grinding surface which surrounds a central aperture in the disc and is in turn surrounded byv a continuous rim. The diameter of the aperture is usually nearly half the outside diameter of the. disc at the rim.

The grinding surface is constituted, by the operative surfaces of several concentric, rings of rectangular nogs, which are separated, as islands, from one another and from the coplanar continuous rim, by approximately concentric channels intersected by straight channels extending outwardly from the central aperture to the rim. Each of the intersecting straight channels is inclined to the radius of the grinder disc drawn from the point at which it starts from the central aperture.

Abrasive fluid, usually a mixture of sand and water, is fed under a hydrostatic head to the central aperture of each grinder disc, from which it floods through the channels and across the operative surfaces of the nogs as the disc rotates, reaching the glass, where it is caught between a face of the glass and the operative surfaces of the respective nogs, finding outlet between the edges of the glass ribbon and the rim of the grinder disc. V

In the case of each bottom disc the abrasive fluid is fed upwardly through the respective tool shaft into the cavity formed within the usual adaptor plate of the respective tool and the I grinder disc, and the hydrostatic head on the abrasive should be such as to maintain full the cavity and the channels whilst the fluid flows to the rim as the disc rotates, so as to assure the maintenance of a requisite film of the abrasive fluid between the operative surfaces of the nogs and the undersurfaceof the glass against;

' maintained full of abrasive fluid.

the pressure exerted by the co-operating upper disc on the glass. f

This hydrostatic head, however, must berestricted so as to avoid any pressure in the central "cavity exceedinga few inches of water pressure.

An excessive pressure in the central aperture tends to raise the glass and prevent the pressure between the glass and thegrinding surface being adequate for grinding. As a consequence of the restricted hydrostatic head, the rate; of flow of the abrasive fluid up the shaft is also restricted,

The hydrostatic head on the abrasive fluidat the bottom of the shaft'is then adjusted to 'produce the normal flow up the shaft and through the channels, while restricting the head in the central cavity. In this way, the central aperture and the channels are, under normal conditions,

The ribbon may be formed by rolling apparatus in front of a glass melting tank furnace in a position predetermined with reference'to the grinding apparatus, but is of variable width, the

width being dependent on several factors, such 'as the height and temperature of the glass" in the tank furnace, which cannot be kept absolutely constant. The ribbon retains its predetermined position for some distance down the lehr, but, by reason of small differences in the rates of cooling at the two sides, it is liable to curve sideways. Extremely small lateral curvature at a point ,in the lehr, say, half or twoethirds of the way down, issuflicient to deflect the position of the ribbon in the grinding and polishing'a'pparatus, ata point some hundreds of feet distant,

by several inches, and bring it into incorrect position relatively to the tools.

The resistance to outward flow of abrasive from each grinder disc of the known form is almost entirely at the rim of the grinder, and

between outflow and feed of abrasive fluid is achieved, but if the superficial area of the circumferential channel exposed is increased by. the ribbon width decreasing, or by lateral movement of. the ribbon, the resistance to mass flow, occurring at the rim, is reduced and engenders ,an excessive flow of fluidfrom the discs.

This leads to an abnormallyhighrate ,of flow of the abrasive fluid from the central cavity,. and

this rate of flow may be higher than the rate of flow up the shaft to the central cavity. Consequently the central cavity becomes partially emptied, the surface of the water atits centre is depressed in paraboloidal form; and the 'air pressure in the cavity is lowered.

As a result of a lowering of air pressure in the central cavity, the glass is sucked downwards and is slightly bent, thus increasingiabnormally the pressur between the glass and the edges of the grinding surface at the periphery of the central cavity. The iron grinding surface at these edges then comes into cohtaet with the f'gla'ss without an interposed film of abrasive fluid 'and the iron of the nogs bears severely and directly on the glass and scores the glass, or causes firing, thereby breaking and/or spoiling glass by causing plate marks, and excessiyewear ing of the inner nogs which ultimately produces aconicity in the grir idi'ng surface requiring removal of the 'gr'inder'di'sc.

,. It is wholly impractical to vafythe'hydrbstatic head o'n the abrasive to ahy'pa tieuiar bottom grinder disc moi-oer to restore the balance between outflow and tees when such excessive flow o ecurs, owing to the large superficial area of a disc, as any increase fluid pressure under the glass acting on 'so great an area waum "lift the glass'ribbon frem the said grinder 'discand separate it from thegri'nding "surface, thereby increasing the pressure on the top'grinder disc and decreasing the pressure on the bottofng'rinder disc and permitting the formation of a layer of sandand water thick enough to prevent eflicient 'r nd 4 r The main meat of the present invention is to pre'vent the occurrence of a suction effect on flat glass in ribbon'f orrn the evento'f a partial vacuum being formed within the eavity of the botn. t 1

In accorfd ance with the present invention a method is devised for proteeuiignpr'rzontairy disaes d fi ass a i 9b ii fad'vaned between co-axial grinding tools operating on both faces ofthe 'glass sirhultaneously omen consists in continually venting the'een'trafcavity or sash r9 2 retie j ei nd fi i' immediately relieving any vacuum produced by ladepressien of water level inthe central cavity and eliminating alsuctiome'ifect'bn the glass. U A bottom grinderconstruotedaccording to the invention comprise a ye -t pi e 'in the oaviity, the vent being disposed near th'e'level of "the grinding surface and the moments the atinosphere being disposed are greater "radius: than A the vent. In suchednstruotionthe vent opening is preferably restrie'ted, as hereafter descr'ibed with 'reretenee 'to the 'a'ccbzfipanyin'g' drawings.

Ihbrder that the iiii'e iitlonfiiay be more larly understood" one lirhdiiieafe'mbodimfit saiiplied to "abottom tool con'ip'risifig a 'gr ndi'n 'a'ise constructed as desoribedirreopnding patehtjapplb cation Serial No. 11'2',f172"wil1how be cribed by way of example'with referenceto the aecoinpanyng 'The bottom toolillustrated comprises "afgrinder disc having -a grinding surface constituted by curve-d bars I2 and 13 or spiral rorm "which are continuously curved "outwardly an -forwardly fully described in the'saia eo pehflm patent apinitiation, and a narrow rim H, the barsbe'ing separated by distributing channels l which mergewith the ends or discoiitiriuousehannfls It which separate the sho'rter'har's [3 from the The disc which is centrally apertured as indicated at H is mounted on the flange IQ of a tool 20 by the intermediary of a casting 2|, usually referred to as an adaptor plate, which with the tool and disc form "a central cavity IIa to which abiasive is'fed ubwariily through the shaft of the tool the mouth of the cavity Ila being constituted by the central aperture I! of the disc.

To prevent a depression of water level being formed in the cavity under the conditions already explained, or if the ribbon breaks, the tool jfl'ang'e I9, is boredand tapped at 23 for the repe eonbrshpgt pipes 24 and 25, the latter communicating with the atmosphere outside the usualsplash guard 26 of the tool. The pipe 2| is"cohhected by-a slightly upwardly inclined pipe g1 to an elbow 28 in which is mounted the vent 29, which elbow may be carried by a bracket 30 mounted on the tool flange l9. It will be observed that the conduit, comprising the parts 29, '18, 21,14, 23, '25, extends downwardly and outwardly from the vent 29 to the connection with the atmosphere.

The vent '29 is disposed near the centre of the "disc (where it is substantially unafl'ected by cencentre of a disc near'the upper surface of the grinding disci. e. in the vicinity-of the unclerface "of a ribbon of glass sup'ported'by the disc, since it is here that the 'air'sp'ace'starts'to form as the water level is depressed, The end part 29 is advantageously'niatie'ofrubb'er or other soft material, in case it comes into contact with the glassfwhen the grinding surface wears.

I The present invention is applicable to the known rormor grinueruisc comprising concenitric rings of nogs each separated from the next as islands and to-anyrorm of grinder disc comprising channels whichfle'ad "the abrasive from a central 'c'avitytoth'e edge of the disc, and the present inVe'htiOnfCOmpriS'eS the combination of venting means 'a'shereindescribed with a bottom "*g'r'ihaerttoiimvmga "grinder disc of either such form, as a means of preventing breakage by scoring and flrinfgf'offthe'glass "when there is an ex- "cess'ive now of museum thedisc.

In ajmodifled ffoi'rn of theinventio'n to prevent delay'in venting the cavity due to the conduit being rd- 1; of, waterand sand, the vent opening 3"! smeare by anapertured plate disposed so that the aperture is co-ax'i'al with the vent, the

rea or the beingicorisrderably less than thecross sectionalarea of the vent. Experiments have shown that an aperture having anarea of about a quarter of'the area-of the sai'd cross sec tion assures the nte'nance 'of a'irp assages to theicavitythroughthesaid conduit. The conduit comprisingthe parts 25, 21, 28 and 29 may be of large enough b'ore everywhere, in relation to the size of 'the 'inltat'the centre of the disc,

to ensure that it never 'nms-run, so that even while sand and water are running out through it; it is possible for airto'be inspired through it in the "opposite direction the'moment'a depression Offlie water level rorms.

Iclaim:

1. A method of protecting horizontally disposed fiat glass as it is continuously advanced between co-axial grinding tools operating on both faces of the glass simultaneously, consisting in admitting atmospheric air into the central cavity of each tool operating on the underface in the vicinity of said under face, thereby immediately relieving any vacuum produced by a depression of water level in the central cavity and eliminating a suction effect on the glass.

2. A grinding tool for grinding the undersurs face of flat glass simultaneously with the grinding of the upper face in which the abrasive fluid is fed to a central cavity in the tool and flows through channels in the grinding surface to the edge of the tool, characterised by a venting conduit leading from the upper part of the central cavity at a point near the axis of the tool through a wall of the cavity to atmosphere at a point distant from the axis, the venting conduit extending downwardly as well as outwardly at all points.

3. A grinding tool according to claim 2 characterised in that the opening in that end of the venting conduit which is situated in the central cavity is restricted with respect to the diameter of the conduit but is coaxial with the conduit at that end thereof, whereby the maintenance of air passage to the cavity through said conduit is assured substantially as described.-

4. A grinding tool according to claim 3, wherein that end of the venting conduit which is situated in the central cavity is disposed near the level of the grinding surface of the tool and is constituted by flexible tubing.

5. A grinding tool according to claim 2, wherein that end of the venting conduit which is situated in the central cavity is disposed near the level of the grinding surface of the tool and is constituted by flexible piping.

6. A bottom grinding tool for grinding the under face of fiat glass in ribbon form as it is continuously advanced in a horizontal plane, said grinding tool having a grinding surface constituted by the upper surface of a series of bars of spiral form curved outwardly and forwardly in the intended direction of the rotation of the disc, distributing channels separating the said bars, a series of peripheral channels connecting the distributing channels in pairs near the periphery of the grinding tool, a central cavity in the tool to which abrasive fluid is fed through the channels, and a venting conduit leading from the upper part of the central cavity at a point near the axis of the tool through a wall of the cavity to atmosphere at a point distant from the axis, the venting conduit extending downwardly as well as outwardly at all points.

JAMES HARRIS GRIFFIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,041,612 Griflin May 19, 1936 

